Cryptosporidiosis currently affects 3-10% of AIDS patients in the U.S. and is considered the leading cause of life threatening diarrhea in these patients. The are no recognized therapeutic measures to combat the disease to date. Hyperimmune bovine anti-Cryptosporidium colostrum has shown clinical benefit in limited studies. Immunoglobulin derived from hyperimmune colostrum has been prepared by ImmuCell for clinical studies under IND 4122. This material is efficacious in vitro and in animal models. Recombinant Cryptosporidium vaccines could be used to provide active immunity in calves (possibly the primary reservoir of human cryptosporidiosis in the U.S.). Such vaccines may also prove useful in early stages HIV patients. Finally, recombinant protective antigens would be expected to elicit much higher levels of protective antibody in the hyperimmune colostral immunoglobulin preparations. Two clones encoding epitopes of sporozoite antigens have been shown to elicit Cryptosporidium neutralizing antibody. These clones, developed at UCSF, are the focus of the current development program that includes optimizing expression, formulation, immunogenicity, and evaluating protection in vitro and in vivo.